The White House has said for the first time that it does not expect to see a climate change bill this year, removing one of the key elements for reaching an international agreement to avoid catastrophic global warming.

In a seminar in Washington, Barack Obama's main energy adviser, Carol Browner, gave the clearest indication to date that the administration did not expect the Senate to vote on a climate change bill before an international meeting in Copenhagen in December.

Browner spoke barely 48 hours after Senate Democrats staged a campaign-style rally in support of a climate change bill that seeks to cut US emissions by 20% on 2005 levels by 2020.

"Obviously, we'd like to be through the process, but that's not going to happen," Browner told a conference hosted by the Atlantic magazine on Friday. "I think we would all agree the likelihood that you'd have a bill signed by the president on comprehensive energy by the time we go in December is not likely."

Browner's bleak assessment deepens concerns that negotiations, already deadlocked, will fail to produce a meaningful agreement in Copenhagen. It also threatens to further dampen the prospects for a bill that was struggling for support among conservative and rustbelt Democrats.

The UN has cast the Copenhagen meeting as a last chance for countries to reach an agreement to avoid the most disastrous effects of warming. Negotiators – including the state department's climate change envoy – admit it will be far harder to reach such a deal unless America, historically the world's biggest polluter, shows it is willing to cut its own greenhouse gas emissions.

Browner's comments undercut a campaign by Democratic leaders in the Senate, corporations and environmental organisations to try to build momentum behind the bill. The day before Browner's comments, John Kerry, the former presidential candidate who is one of the sponsors of the cap-and-trade bill, told a conference he remained confident the bill would squeak through the Senate.

Her remarks also raise further doubts about how forcefully the Obama administration is willing to press the Senate for a climate bill in the midst of its struggles over healthcare.

In the last two weeks, diplomats have grown increasingly frustrated with the administration. Negotiators say they understand Obama would have to struggle to get this agenda through the Senate, but say the president has shied away from opportunities to make the case for climate change.

Obama came in for harsh criticism from environmental organisations for failing to urge the Senate to act during a speech to the United Nationssummit on climate change late last month. Environmental groups called it a "missed opportunity".

"If there is no serious US legislation in place then we will have delegations arriving and getting increasingly frustrated with nothing happening," said John Bruton, the European Union's ambassador.